Notes 2.0

Repurposing this ticket to add more information on field paths and array behavior. Suggestion is to link the field path section in the aggregation quick reference page to a larger section on field paths (potentially a new page). Then, link the new field paths section/page to operator pages that might have use cases for field paths . 

Notes 1.0

See below. It seems like we could highlight this behavior by linking to https://www.mongodb.com/docs/manual/tutorial/query-array-of-documents/, perhaps in an admonition.

Description

Currently the $arrayElemAt documentation does not include an example of how it behaves when dot notation is used to reference a nested field name within an array.

A simplified example:

With the following data:

{
	"_id" : ObjectId("5ba53172ce6fa2fcfc58e0ac"),
	"example" : [
		{
			"apples" : [
				"macintosh",
				"golden delicious",
				"fuji",
				"gala"
			]
		},
		{
			"oranges" : [
				"mandarin",
				"navel",
				"clementine"
			]
		},
		{
			"apples" : [
				"braeburn",
				"honeycrisp",
				"red"
			]
		}
	]
}

You can include the nested field "apples", and the index value will determine whether you get the first entry in the "example" array, or the third:

db.fruits.aggregate([
   {
      "$project": {
         "apples2": {
            "$arrayElemAt": ["$example.apples", 1]
         }
            }
   }
])

This returns the second array that matches the "apples" field name:

{
	"_id" : ObjectId("5ba53172ce6fa2fcfc58e0ac"),
	"apples2" : [
		"braeburn",
		"honeycrisp",
		"red"
	]
}

Instead of simply returning the second nested field in the "example" array ("oranges"), which would be returned if the 1 index was used without specifying a nested field:

db.fruits.aggregate([
   {
      "$project": {
         "apples2": {
            "$arrayElemAt": ["$example", 1]
         }
            }
   }
])

returns:

{
	"_id" : ObjectId("5ba53172ce6fa2fcfc58e0ac"),
	"apples2" : {
		"oranges" : [
			"mandarin",
			"navel",
			"clementine"
		]
	}
}

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)